Previously, typical methods for continuously producing a large amount of optical information media having a micro-protrusion/depression structure have included a “pressing method” described in Japanese Patent No. 4190473 (PTL1), a “casting method” described in Japanese Utility Model Registration Laid-Open No. 2524092 (PTL2), a “photopolymer method” described in Japanese Patent No. 4088884 (PTL3), and the like.
In the case where the micro-protrusion/depression structure is produced by the “pressing method”, the micro-protrusion/depression structure is shape transferred by heating a resin layer which has been formed as a continuous layer to the softening point or higher and pressing a relief mold (a mold for reproduction of the micro-protrusion/depression structure) against the face of the resin layer where the micro-protrusion/depression structure will be formed. Alternatively, the micro-protrusion/depression structure may be shape transferred by pressing to a resin layer a relief mold which has been heated to the softening point of the resin layer or higher. In either method, a technique of shape transfer is utilized, which involves pressing the relief mold to the resin layer which has been previously formed by whole surface coating or the like. Further, addition of colorants such as dye or pigment allows to color the gloss obtainable after providing a metal reflective layer. However, the pressing method is predicated on the presence of the colored resin layer throughout the whole surface of pressing processing
In the “casting method”, the shape of the micro-protrusion/depression structure is transferred by melt extruding a resin for forming the micro-protrusion/depression structure which is heated to its melting point or higher onto a relief mold (a mold for reproduction of the micro-protrusion/depression structure), or by casting a solution or dispersion of the resin onto the relief mold. The micro-protrusion/depression structure is obtained by cooling the resin to decrease its flowability to form a continuous layer, and peel it off from the relief mold. Also in this case, coloration is possible by adding a colorant such as dye or pigment to the resin layer. However, similar to the “pressing method”, the colored resin layer exists as a continuous layer.
The “photopolymer method” (a 2P method, or a photosensitive resin method) comprises the steps of casting a radiation-curing resin composition between a “relief mold (a mold for reproduction of the micro-protrusion/depression structure)” and a flat substrate (such as a plastic film), curing the resin composition by the radiation to form a continuous layer, and peeling off the cured resin layer as well as the substrate from the relief mold. The micro-protrusion/depression structure of high definition can be obtained by utilizing the “photopolymer method”. The optical information medium obtained by the “photopolymer method” has a superior precision in formation of the protrusion/depression structure, high thermal resistance, and high chemical resistance, compared with those obtained by the “pressing method” and “casting method” in which the thermoplastic resin is used. Further, heating is unnecessary during processing, since the radiation-curing resin composition in a liquid form is used.
However, the following problem is present in any of the molding methods of “pressing method”, “casting method”, and “photopolymer method”. In any of the molding methods, the resultant resin layer is obtained as a continuous and unitary layer. For example, it is difficult to continuously duplicate a large amount of the micro-protrusion/depression structures disposed only in desired regions of a resin layer which are not unitary but consist of multiple parts. Also, it is difficult to duplicate continuously a large amount of the micro-protrusion/depression structures disposed only in desired regions of a resin layer which is partially colored. In regard to this point, it might be conceivable to form the micro-protrusion/depression structure by disposing a colored resin layer consisting of multiple parts onto a supporting substrate and adjusting the position where the relief mold is pressed on, in the “pressing method”. However, in the case where a resin layer consisting of multiple parts is present, the productivity is reduced in view of register. This is because heat-shrinkage of the supporting substrate becomes uneven.
One of optical information media is an optically variable device (OVD) as, which can be used as media for the purpose of decoration or anti-counterfeiting. Diffraction gratings and scattering structures are mainly used as the micro-protrusion/depression structure in the OVD. Generally, any of the “pressing method”, “casting method” and “photopolymer method” are used for forming such micro-protrusion/depression structure. The OVD which has undergone a vapor deposition step for disposing a reflecting layer on the micro-protrusion/depression structure and a coating step for disposing an adhesion layer may be in a form of a transfer leaf, an adhesion label, a thread, or the like.
A unique metallically glossy color can be provided on this OVD by coloring the resin layer constituting the micro-protrusion/depression structure by colorants such as a dye or a pigment. For example, the OVD has a silver metallic gloss when a colorless resin layer is used and a reflective layer is made of aluminum. On the other hand, even in the case where the reflective layer is made of aluminum, the OVD exhibits gold color when the resin layer has been colored in orange (or yellow), or the OVD exhibits copper color when the resin layer has been colored in reddish-brown.
In order to provide higher resistance to counterfeiting, higher design properties and higher chemical resistance, it is possible to subject the OVD to a demetallization treatment. Demetallization treatment generally means a method comprising the steps of: providing a mask layer having a desired pattern onto a reflective layer made of metal, etching and partially remove the metal with an acid or an alkali to obtain the reflective layer having the desired pattern. For example, demetallization-treated holograms are often provided to securities such as bank notes.
However, the following problem exists in the case where the OVD is produced with the demetallization treatment. If the resin layer constituting the micro-protrusion/depression structure is colored in a certain color, the part of the resin layer where the metal is removed by the demetallization treatment is not colorless under visual observation but exhibits the certain color. For example, the demetallized part which is desired to be colorless exhibits orange color, when an OVD exhibiting gold metallically glossy color is produced with the demetallization treatment. Therefore, gold gloss of the non-demetallized part cannot be emphasized to the general public who is a judge of genuineness, since the reflective layer does not appear to be made of gold metal. Further, it is also problematic that the same appearance can be achieved by forming an orange coating onto the whole surface of the counterfeited hologram.
It might be conceivable to make the reflective layer from metal having non-silver gloss, against the above problem. For example, if the micro-protrusion/depression structure is formed with a colorless resin layer, and copper is vapor deposited thereon instead of aluminum, and demetallization treatment is carried out to obtain an OVD, the region where the copper is removed becomes colorless, since copper itself has copper-colored gloss.
However, this production method suffers from the following problems. In appearance, the chemical resistance of copper, including resistance to human sweat, is inferior to that of aluminum whose oxide is white, since oxides of copper are colored. Therefore, copper is not a practical material in the actual distribution. Further, in a method for providing the reflective layer by vapor deposition of copper, only copper-colored reflective layer can be made, unless one or more steps of vapor deposition of metal of other color, one or more masking steps, and/or one or more etching steps are added. For example, vapor deposition and demetallization treatment of gold for providing gold gloss is very expensive and exhibits low productivity compared to the case where the resin layer having the micro-protrusion/depression structure is colored.
In regard to these problems, it is possible to adopt a method of staining a part of a layer which has been formed on the substrate before the resin layer having the micro-protrusion/depression structure. For example, a colored part having a desired pattern can be provided on a surface of the resin layer having the micro-protrusion/depression structure, the surface being opposite to the reflective layer (that is, between a peel layer and the resin layer having the micro-protrusion/depression structure in the hologram transfer leaf). Thus, discontinuous and patterned glossy expression exhibiting a color other than silver is achieved by the colored part and the reflective layer. However, this method suffers from the following problem. It is necessary to form the colored part before formation of the resin layer having the micro-protrusion/depression structure in this method. Therefore, in any of the “pressing method”, “casting method”, and “photopolymer method”, the productivity is reduced in view of register.
Against these problems, a self-alignment patterning is proposed, which is based on difference in transmittance of light of the metallic reflective layer caused by structural difference of the resin layer having the micro-protrusion/depression structure. For example, a moth-eye structure of sub-wavelength scale is introduced into a part of the micro-protrusion/depression structure, and aluminum is vapor deposited onto the micro-protrusion/depression structure. Here, the layer of the vapor deposited aluminum has a relatively small thickness on the part of the moth-eye structure, since the part of the moth-eye structure has a large surface area compared to other part. Then, a positive-working photolithographic material is further coated onto the reflective layer, as a mask layer, and the mask layer is irradiated with a light for making the photolithographic material soluble, from the side of the micro-protrusion/depression structure-formed layer. In the part of the moth-eye structure, the positive-working photolithographic material becomes soluble, since the thickness of aluminum is small to transmit the light therethrough. In the other part, such as the part not having the moth-eye structure or a diffraction grating, the positive-working photolithographic material does not become soluble, since the light is not transmitted. The colored part can be provided in a self-alignment manner by washing off the solubilized photolithographic material. However, this method suffers from the following problem. In this production method, the colored part is not provided on the front side of the general optical information medium (that is, the side of the resin layer having the micro-protrusion/depression layer), rather it is provided on the back side (that is, on the side of the reflective layer opposite to the resin layer). Therefore, genuineness determination is possible by visual observation from the back side. It is impossible to stain the resin layer having the micro-protrusion/depression structure which is positioned on the front side.
As described above, it is impossible to continuously produce a large amount of demetallized optical information media for observing a reflective layer from the side of the micro-protrusion/depression structure-formed layer, wherein coloration is limited only in the region where the reflective layer is present, and the region where the reflective layer is absent (that is, demetallized part) is colorless, equal to or more than the extent of the conventional demetallized optical information medium having uniform color (including colorless)